This invention relates generally to the field of optoelectronic devices. More specifically, this invention relates to compound semiconductor structures for optoelectronic devices such as light-emitting diodes, photodetectors, edge-emitting lasers, and vertical-cavity surface-emitting lasers.
Semiconductor devices operating at 1.3 xcexcm and 1.55 xcexcm are extremely important for fiber-optic communications. Ideally the devices for these applications should operate at a single wavelength, be robust to environmental variations such as temperature, and be inexpensive to produce. To date, most work has concentrated on producing edge-emitting devices using the InP/InGaAsP material system. These devices employ special distributed feedback structures to control the spectral quality of the laser output. However, the yield of these devices can be poor. Furthermore, because these devices are made of smaller bandgap materials grown on InP, they are highly temperature-sensitive and require strict temperature control. Therefore this type of long-wavelength edge-emitting laser is usually very costly.
An alternative device which may allow single wavelength emission is a vertical-cavity surface-emitting laser (VCSEL). In general, VCSELs are light-emitting semiconductor devices comprising two distributed Bragg reflectors (DBRs), between which lies an active region composed of a material emitting the desired wavelength of light. The DBRs act as mirrors, and define a resonant cavity, and the active region acts as an optical gain medium. There may also be spacers between the active region and each DBR used to define a cavity length. The semiconductor mirror structures are usually doped to allow current flow through the active region.
There are problems associated with prior art VCSELs, some of which have been reviewed in U.S. Pat. Nos. 5,719,894 and 5,719,895 to Jewell et al., the disclosures of which are incorporated herein by reference. In general, the production of VCSELs grown using InP/InGaAsP and emitting in the region of 1.3 xcexcm to 1.55 xcexcm has been inhibited because of the high thermal sensitivity and poor refractive index properties of the InP/InGaAsP system. In addition, the production of efficient DBRs for InP substrates is difficult and in practice has been found to be very ineffective.
To overcome the production of poor quality mirrors based on InP, one approach has been to use wafer fusion. In this technique, the active region is grown on an InP substrate and the DBRs are grown on gallium arsenide (GaAs). These wafers are then processed and bonded together under high pressure to form a VCSEL. The drawbacks of this method are possible reliability issues because of the complex processing required and the attendant higher manufacturing cost.
To overcome the limitations of InP/InGaAsP, structures based on GaAs substrates have been proposed for vertical cavity devices. The growth of high quality active material on GaAs for 1.3 xcexcm and 1.55 xcexcm emission is a problem which has been investigated using a number of different approaches.
A first approach uses InGaAs quantum dots (QDs) grown on GaAs. This approach has produced photoluminescence (PL) at 1.3 xcexcm, a resonant cavity photodiode operating at 1.27 xcexcm, and an edge emitting quantum dot (QD) laser operating at 1.3 xcexcm. A continuous-wave (CW), room temperature (RT), QD-based VCSEL has also been produced, but the lasing wavelength was only 1.15 xcexcm.
A second approach uses strained GaAsSb quantum wells (QWs). This approach has produced room temperature PL at 1.3 xcexcm and an edge-emitting laser operating at 1.27 xcexcm. (The shorter wavelength of this laser can be attributed to gain saturation at the higher current injection levels due to the limited number of defect-free QWs which can be grown.) This approach has also produced PL wavelengths of up to 1.332 xcexcm using GaAsSb/InGaAs bi-layer QWs, with a type-II band-edge alignment.
A third approach uses a single GaInNAs quantum well. This approach has produced room-temperature pulsed operation at an emission wavelength of 1.18 xcexcm with a threshold current density of 3.1 kA/cm2. A CW edge-emitting laser having a lasing wavelength close to 1.3 xcexcm has also been produced when the nitrogen content of the QW is increased to 1%. Threshold currents of 108 mA have been achieved for devices with a cavity length of 800 xcexcm and an active width of 2 xcexcm.
These approaches all have shortcomings. First, the wavelengths produced are too short for telecommunications purposes. Second, the quantum dot devices rely on long cavities and use highly reflective facet coatings. Third, the GaInNAs approach is limited because the incorporation of nitrogen in InGaAs to form GaInNAs is technologically challenging for a number of reasons. First, there are problems in reliably incorporating more than 1% nitrogen in the active material. Second, a typical precursor used is based on hydrazine (e.g. rocket fuel), and great care must be taken because of the unstable and pyrophoric nature of the compound. Third, it is not clearly understood how the nitrogen is incorporated into the active region. Although some researchers previously thought that a quaternary alloy is formed, it is now generally believed that nitrogen is incorporated as an impurity or defect state. Such states can introduce non-radiative recombination centers which increase in number as the amount of nitrogen incorporated into the material increases. These states may cause local perturbation, or splitting of the conduction band, allowing longer-wavelength emission to be achieved. However, higher nitrogen incorporation generally shortens device lifetime, consistent with the introduction of defects.
Therefore, a need has arisen for improved semiconductor optoelectronic devices that operate at the desired telecommunications wavelengths of 1.3 xcexcm and 1.55 xcexcm.
In accordance with the present invention, a compound semiconductor device is provided that includes a substrate and an active region disposed above the substrate. The active region includes at least two different pseudomorphic layers, the first layer having the form InxGa1xe2x88x92xPyAszSb1xe2x88x92yxe2x88x92z, and the second layer having the form InqGa1xe2x88x92qPrAssSb1xe2x88x92rxe2x88x92s. The first layer includes at least In, Ga, and As, and the second layer includes at least Ga, As, and Sb. xe2x80x9cPseudomorphicxe2x80x9d is defined as having a sufficiently low level of misfit dislocations. Each InGaPAsSb layer is pseudomorphic to the substrate. The substrate is preferably GaAs or AlpGa1xe2x88x92pAs (0 less than p less than 1), or of a material having a lattice constant close to or equal to that of GaAs. For the first layer, it is preferable if x is between 0.05 and 0.7, y is between 0 and 0.35, z is between 0.45 and 1, and 1xe2x88x92yxe2x88x92z is between 0 and 0.25. For the second layer, it is preferable if q is between 0 and 0.25 and 1xe2x88x92rxe2x88x92s is between 0.25 and 1.
Preferably, the band structure formed between the first and second layers has a type-II band-edge alignment. Preferably, the peak transition wavelength is greater than 1100 nm.
Preferably, the first layer is a well region for electrons, and the second layer is a barrier region for electrons. Preferably, both layers form quantum wells and may also form a superlattice.
In another embodiment, the active region further includes a third pseudomorphic layer. This third layer has substantially the same composition as the first layer and may be disposed on the second layer. A variation of this embodiment also includes at least one layer-pair between the second and third layers. Each layer-pair has substantially the same composition as the first and second pseudomorphic layers. Another variation includes a fourth pseudomorphic layer disposed on the third layer, the fourth layer having substantially the same composition as that of the second layer. This variation could also have at least one layer-pair between the second and third layers, each layer-pair having substantially the same composition as the first and second pseudomorphic layers.
In another embodiment, the active region could further include three additional pseudomorphic layers disposed above the second layer. The first additional layer has a composition different from that of either of the first two layers. The second additional layer has substantially the same composition as that of the second pseudomorphic layer, and the third additional layer has substantially the same composition as that of the first pseudomorphic layer.
In another embodiment, cladding layers surround the active region. Preferably, these layers are made of GaAs, AltGa1xe2x88x92tAs, or GaAsuP1xe2x88x92u, where t and u are between 0 and 1.
In another embodiment, the first pseudomorphic layer is disposed on the first cladding layer, and the second pseudomorphic layer is disposed on the first pseudomorphic layer. Preferably, this embodiment includes a third pseudomorphic layer, having substantially the same composition as the first layer, and disposed above the second layer. A variation of this embodiment may also include at least one layer-pair between the second and third layers. Each layer-pair has substantially the same composition as the first and second pseudomorphic layers. Another variation includes a fourth pseudomorphic layer disposed on the third layer, the fourth layer having substantially the same composition as that of the second layer. This variation could also have at least one layer-pair between the second and third layers, each layer-pair having substantially the same composition as the first and second pseudomorphic layers.
In another embodiment, a first cladding layer is disposed between the substrate and the active layer, and a second cladding layer is disposed above the second pseudomorphic layer. The second cladding layer and the layers disposed between the first and second cladding layers then may form a multilayer structure which could repeatedly be disposed over the initial multilayer structure.
In a further embodiment, there are two conductivity layers electrically coupled to the active region, one of a first conductivity type and the second of another conductivity type. There is also means for providing or extracting electrical current to or from the active region. Preferably, the bandgap of the conductive layers is larger than that of the layers of the active region. Preferably, an edge-emitting device is formed in which a cavity in the plane of the conductive layers forms a semiconductor-air interface through which optical emission or absorption is achieved.
In another embodiment of this device, there is a grating layer disposed above the second conductive layer. The grating layer has lines that extend over at least part of the active region and the grating layer defines an optical resonance cavity. The cavity has a resonance wavelength related to a resonance energy such that the resonance wavelength (in microns) equals 1.24 divided by the resonance energy (in eV). Preferably, the grating layer lines can be shifted by at least a quarter-wavelength, or a multiple of a quarter wavelength, to form a phase-shifted grating layer.
In another embodiment of this device, a bottom mirror is disposed beneath the active region and a top mirror is disposed above the active region. The top and bottom mirrors define an optical resonance cavity having a resonance wavelength related to a resonance energy such that the resonance wavelength (in microns) equals 1.24 divided by the resonance energy (in eV). Preferably, the top and bottom mirrors are made of alternating high refractive index and low refractive index layers. The low refractive index layers can be made of oxidized material, low refractive index dielectric material, low refractive index polymeric material, and relatively low refractive index semiconductor material, or any combination of these. The high refractive index layers can be made of oxidized material, high refractive index dielectric material, high refractive index polymeric material, and high refractive index semiconductor material, or any combination of these.
In a further embodiment, there is an aperture disposed above the active region. The aperture has two regions. In one further embodiment, one aperture region has high electrical resistance and the other aperture region has a much lower electrical resistance. In another embodiment, one aperture region has a lower refractive index than the other aperture region. In another embodiment, the first aperture region is made of predominantly oxidized material and the other aperture region is less oxidized than the first aperture region. In another embodiment, the aperture is formed by etching a pillar.
Also in accordance with the present invention are an edge-emitting laser, a resonant cavity photodetector, a resonant cavity light-emitting diode (LED), or a VCSEL each including a substrate with an active region disposed above the substrate. In each device, the active region includes at least two pseudomorphic layers. The first pseudomorphic layer has the form InxGa1xe2x88x92xPyAszSb1xe2x88x92yxe2x88x92z, the second pseudomorphic layer has the form InqGa1xe2x88x92qPrAssSb1xe2x88x92rxe2x88x92s, and the compositions of the first and second pseudomorphic layers are different. The first layer includes at least In, Ga, and As, and the second layer includes at least Ga, As, and Sb.
Also in accordance with the present invention is a compound including indium, gallium, phosphorus, arsenic, and antimony and having the form InxGa1xe2x88x92xPyAszSb1xe2x88x92yxe2x88x92z, in which 0 less than x less than 1,0 less than y less than 1,0 less than z less than 1, and 0 less than 1xe2x88x92yxe2x88x92z less than 1
Also in accordance with the present invention is a method for fabricating a compound semiconductor device on a substrate including forming an active region disposed above the substrate, the active region including at least first and second pseudomorphic layers. The first pseudomorphic layer has the form InxGa1xe2x88x92xPyAszSb1xe2x88x92yxe2x88x92z, the second pseudomorphic layer has the form InqGa1xe2x88x92qPrAssSb1xe2x88x92rxe2x88x92s, and the compositions of the first and second pseudomorphic layers are different. The first layer includes at least In, Ga, and As, and the second layer includes at least Ga, As, and Sb. The substrate is preferably GaAs or AlpGa1xe2x88x92pAs (0 less than p less than 1), or of a material having a lattice constant close to or equal to that of GaAs.
By using two differently composed InGaPAsSb pseudomorphic layers in the active region, the present invention avoids the limitation of the lasing wavelength being determined by the bandgap of a single material. Wavelengths from 1.1 xcexcm to 1.5 xcexcm have been achieved. Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims.